New applications of mobile communications demand high-speed and high-quality, bandwidth-efficient wireless access solutions. The application of MIMO (multiple antennas both in the transmitter and in the receiver) has been demonstrated to drastically improve channel capacity compared to single-antenna systems. On the other hand, OFDM has demonstrated its high spectral efficiency and ability to deal with frequency selective fading and narrow band interference. Therefore the combination of OFDM with spectrally efficient multiple antenna techniques opens the door to high data-rate wireless communication.
Compared with the single input single output (SISO) systems, two kinds of gains are provided by the MIMO wireless systems, namely diversity gain and multiplexing gain. With diversity gain more reliable reception can be realized. With multiplexing gain the capacity of MIMO systems increases linearly with the number of transmit and receive antennas. This is due to the fact that a rich scattering environment can provide multiple data pipes within the same frequency band by using techniques such as space-time coding and space-time layering. Since the capacity can be potentially increased by the application of multiple antennas, the use of up to four antennas at the transmitter and/or receiver has been considered to achieve an increased data rate for a given link performance criterion, or to improve link performance for a given data rate.
For wireless propagation environments, the inherent temporal and spatial variations of wireless channels impose more challenges on the design of a reliable communication system. For noise and interference limited systems, coherent demodulation can achieve 2.5-3 dB SNR gain compared to the differential demodulation. When coherent detection is performed in a receiver, reliable channel estimation is very important to the system performance. Channel estimation in MIMO systems is more complicated because multiple channels should be obtained individually. As the number of transmit antennas increases, the sensitivity to any channel estimation error becomes more pronounced.
OFDM modulation has been adopted by several standards, such as DVT-T, IEEE802.11a/g and IEEE802.16a/d. Different training schemes have been employed in these standards, including preamble, fixed-location pilot and variable-location pilot. However MIMO is not mandatory and is only adopted by IEEE802.16a as optional, and only two transmit antennas on the base station side and one receive antennas on the SS (subscriber station) side are employed. Since IEEE802.16a is designed for fixed and portable applications, the channel varies slowly. For the Wireless MAN (metropolitan area network) OFDM air-interface, the channel estimation is obtained from the preambles. For the Wireless MAN OFDMA air-interface, although variable location pilot symbols are introduced, they are only used to update the channel slowly.